Certain industries, for instance the entertainment, architectural or theater industries, have applications for specialized lighting which can benefit from an apparatus or system which is able to produce colors selected from among a palette of an extremely large number of colors, and which is able to control the direction at which the light is projected. A palette having millions of colors is useful for applications such as light painting, product enhancement, and special effects.
The color of light emitted from a light source is determined by its spectral properties. The spectrum can be duplicated by a weighted sum of the additive primary colors red, blue and green. A single color can be produced by an individual light emitting diode (LED), the color being either a primary color or a color which is a composite of more than one primary color. LEDs can be produced having a variety of colors. A composite emitted light can be made by grouping LEDs of various combinations of colors in close physical proximity, with each LED individually emitting at a selectable intensity. The LEDs may also be placed in a reflective cavity that is shaped to enhance control over the direction of the composite light. The composite light may be used, for instance, for artistic, theatrical, or display purposes. However, light from the individual LEDs historically has been difficult to collimate to a narrow beam, thereby producing a composite light having poor color uniformity. Collimation and beam width are related terms, in which a highly collimated beam necessarily is a beam that has a narrow beam width compared to a beam that is not highly collimated.
A directed light beam is light emitted in a preferred direction, and can be characterized by beam angle and dispersion. Beam angle refers to the full beam dispersion angle at half the maximum on-axis luminous intensity. Intensity dispersion is a measure of the distribution of light over an angle with respect to the center of the light beam. Specialized lighting applications such as those identified above can benefit from having the ability to project a directed light beam of a composite of colors over a long distance. The distance of projection is increased when the emitted light is concentrated into a small beam angle.
FIG. 1 is a top view of LED placement locations within a conventional light engine cavity, in which “B” indicates a blue excitation emitter with wavelength 440-495 nm, “R” indicates a direct emission red, orange, or amber with wavelength range 575-680 nm, and “G” indicates a direct emission green wavelength having a range 495 nm-575 nm. The LEDs are typically mounted on a substrate 1 which provides electrical connections, thermal dissipation, and mechanical support.
LED spacing within the light engine limits the minimum distance at which the light engine can be located from the target of its illumination, because too small a distance from the target of illumination produces poor composite color uniformity illumination of a close-in target. Typical spacing between the individual LEDs is approximately 0.2032 millimeters as shown but may vary by as much as ±0.5 mm or more. Color mixing improves as LED spacing is reduced, but equipment or speed of manufacture limit how close together the LEDs may be placed, causing conventional multi-colored light engines like that shown in FIG. 1 to suffer from poor color mixing.
Light engines are designed with the LEDs spaced relatively widely apart for improved heat dissipation, thereby causing poor color mixing. Viewers may see the poor color mixing as changes in the perceived light color from the light engine when viewed from different viewing angles. Optical devices for controlled color mixing developed by the applicant are known and described in commonly-assigned U.S. patent application Ser. No. 11/737,101, the entire content of which is incorporated by reference herein in its entirety. Second, fabrication machines and techniques may limit the minimum distance the LED die can be placed on the substrate.
Light from an emitter like that of FIG. 1 is conventionally passed through a rotationally-symmetric passive optic collimator in order to control the direction of light rays emitted by the engine. FIG. 2 is an illustration of the close-in beam illuminance pattern resulting from passing the light emitted by the light engine of FIG. 1 through a rotationally symmetric total internal reflection (TIR) secondary optical lens. The illuminated area does not have the desired uniformity of illumination, but instead has multiple colors illuminated. The red, green and blue primary colors emitted by the individual LEDs are focused in different locations in the field. The area of poor color uniformity may include any non-desired combination of colors emitted by the individual LEDs, and may be in any portion of the illuminated area, and the region may be of any shape. This separation of the colors is not desirable for some applications.
The conventional solutions to collimating multi-primary emitters produce a more homogeneous color uniformity at the expense of a wider beam width, and therefore the conventional solutions cannot separately and simultaneously optimize both color uniformity and beam width. In addition, for some lighting applications, e.g., entertainment applications, there is a need to “throw” or project a selected color at a screen or surface at a distance of ≧15 meters while maintaining an acceptable level of illumination and color uniformity. High illuminance at a long throw distance requires a narrow beam. Light intensity dispersion must be minimized in order to maximize the throw distance. Therefore, a need exists to provide an optics assembly which can simultaneously optimize the collimation and color uniformity of a light beam produced by a light engine.